Was Einstein Right? The Binary Pulsar and Gravity Waves

The binary pulsar Taylor and Hulse discovered in 1974 gave physicists something they had been looking for since 1915 -- a way to test whether Einstein was right about gravity waves. Much like vibrating electrons give off electromagnetic radiation (radio waves for instance), Einstein's general theory of relativity predicts that an accelerating mass gives off energy as gravitational radiation, which can be thought of as ripples moving at the speed of light in the curvature of space-time. Compared to other kinds of radiation, however, gravity is very weak, and even Einstein questioned whether these waves would ever be detected.

The two stars of the binary pulsar, each about 1.4 times the mass of the sun, are locked in close orbit with each other, creating a very strong gravitational interaction. The resulting gravity waves, according to the theory, should measurably reduce the orbital energy. In other words, much as the orbit of an Earth satellite gradually decays closer and closer to Earth -- because it loses energy to the atmosphere -- the pulsar and its companion should gradually spiral toward each other and orbit faster and faster.

Detailed astronomical measurements by Taylor and his colleagues confirm that this is the case. The interval between radio pulses from the pulsar is getting about 75 millionths of a second shorter each year, almost precisely what general relativity predicts should happen. This is the strongest evidence to date that gravity waves exist.